Photochemistry of the 1,6-Dideuterio-1,3,5-hexatrienes in Solution: Efficient Terminal Bond Photoisomerization in One-Bond-Twist and Bicycle Pedal Ways

Abstract

The report that the central bond photoisomerization of the 1,3,5-hexatrienes (Hts) is highly inefficient has encouraged theoreticians to seek conical intersections (CIs) at geometries that can explain rapid nonradiative return to the initially excited isomer. Because they are photochemically silent, torsional relaxations about the terminal double bonds of the Hts have not been evaluated as significant radiationless decay pathways. Study of the photoisomerization of trans,trans,trans- and trans,cis,trans-1,6-dideuterio-1,3,5-hexatrienes (ttt- and tct-Htd2) addresses this issue. Degassed cyclohexane-d 12 (C6D12) and CD3CN solutions were irradiated at 254 nm in quartz NMR tubes, and the progress of the reactions was followed by 1H NMR. Photoisomerization rates based on the integration of terminal hydrogen NMR peaks are in reasonable agreement with rates obtained by fitting pure isomer NMR spectra to the phase shift and baseline corrected experimental NMR spectra. The results show that terminal bond isomerization is highly efficient, especially when one considers that central bond isomerization is much more efficient than previously reported and is mainly observed together with terminal bond isomerization. A mechanism involving terminal one-bond-twist (OBT) in competition with a bicycle pedal (BP) process accounts for all terminal and most central bond photoisomerization. OBT central bond isomerization is a minor reaction that is observed primarily in the tct to ttt direction. Most surprising is the prominent role of the BP process in central bond photoisomerization. Proposed initially to account for photoisomerization in free volume constraining media, it is observed here in the absence of medium constraints.

Photochemistry and photophysics of cholesta-5,7,9(11)-trien-3β-ol: a fluorescent analogue of cholesterol

“…By comparison, their diene precursors, Pro and ergosterol, fluoresce only in low T glassy media [47][48][49][50] and undergo no intersystem crossing [46]. The unusual behavior is no doubt due to the structural rigidity that inhibits the torsional relaxation pathways that dominate the photochemistry and radiationless decay of the parent 1,3.5-hexatrienes [51]. It is reminiscent of the dramatic enhancement of fluorescence that structural rigidity induces on the trans-stilbene chromophore [52].…”

Evidence of the Torsion of a Polyene Chain in a Strongly Hindered Molecular Environment: The Ttbp4 Crystal

“…While it is not our aim to study the mechanism through which the molecule rotates, we want to note that there are different mechanisms proposed in the literature to explain torsion in condensed phase, such as one-bond flip (OBF, also known as the double-bond twist), volume-conserving hula-twist (HT) and the bicycle pedal (BP) mechanisms [35]. Some examples of this torsions in in condensed phases are the BP mechanism observed in crystalline butadientes [36][37][38][39], hexatrienes [40,41] and photoactive yellow protein [42] and has been implicated in the visual retinoid cycle [11].…”

α-Methylstilbene Isomers: Relationship of Structure to Photophysics and Photochemistry

“…Our success in growing X-ray-diffraction-quality cis - and trans -α-methylstilbene crystals was a main motivation for this study: first, because ours is the first crystal structure of a cis -stilbene derivativethe parent cis -stilbene is a liquid at room temperatureand second, because trans -stilbene has been shown to undergo dynamic bicycle pedal (BP) equilibration in the solid state. , Volume conserving simultaneous rotation about the two essential phenyl-vinyl single bonds in the ground state of trans -stilbene is the first experimental demonstration of the analogous bicycle pedal motion proposed by Warshel for the excited retinyl moiety in the protein cavity of rhodopsin . Experimental demonstrations of BP excited-state isomerizations in crystals − and low-temperature glassy media as well as in solution have appeared. Methyl substitution along conjugated polyene moieties is ubiquitous in nature given the important role of isoprene units in biomolecules.…”

“…16,17 Volume conserving simultaneous rotation about the two essential phenyl-vinyl single bonds in the ground state of trans-stilbene is the first experimental demonstration of the analogous bicycle pedal motion proposed by Warshel for the excited retinyl moiety in the protein cavity of rhodopsin. 18 Experimental demonstrations of BP excited-state isomerizations in crystals 19−23 and low-temperature glassy media 24 as well as in solution 25 have appeared. Methyl substitution along conjugated polyene moieties is ubiquitous in nature given the important role of isoprene units in biomolecules.…”